The landmark lOM report, "Emerging Infections: Microbial Threats to Health in the United States" highlighted the importance of studying Infectious diseases in natural animal hosts-60% of all human pathogens are directly transmitted from or emergent from animal reservoirs. Understanding the behavior of pathogens in their natural hosts, including mechanisms of persistence, evolution of virulence, acquisition and maintenance of antimicrobial resistance, and genetic change underlying transmission phenotypes, is now widely recognized as critically important to addressing emerging infections. The broad spectrum of microbial pathogens, from small RNA viruses to multicellular parasites, which infect domestic and wild animals, provides a rich opportunity to understand pathogen behavior in their natural hosts both as models for human disease as well as for directly understanding pathogen evolution and transmission to humans. Our program specifically and uniquely addresses this need and integrates clinically trained veterinarians with post-PhD fellows to provide a rigorous and dynamic training environment. Prominent examples of T32 trainee success in the past 10 years include: i) 7 trainees transitioned to individual K08 (post-DVMs) or F32 (post-PhD) awards;ii) 10 post-DVM trainees received the PhD (4 remain in training);and iii) all past trainees continue in research and there is clear evidence of progressive career advancement. The program has two types of trainees: (i) clinically-trained veterinarians, usually with residency training in clinical microbiology, internal medicine, or pathology;and (ii) post-PhD fellows who expand and enhance their research capabilities in infectious diseases. Critically, both groups of trainees are integrated in the laboratory research training phase with shared trainee led research-in-progress seminars, shared trainee led outside speaker seminars, and in the responsible conduct of research progression. This integration significantly enhances the training environment as each type of trainee brings unique strengths. We propose to continue training 5 fellows per year (3 post-DVM;2 post-PhD) with a minimum of 3 years of dedicated laboratory research for the clinically trained fellows and at least 2 years for post-PhD fellows.